High energy solid propellants containing fluoropolymers and metallic fuels



United States Patent 3,351,505 HIGH ENERGY SOLID PROPELLANTS CON- TAINING FLUOROPOLYMERS AND METAL- LIC FUELS Isadore Shapiro, Beverly Hills, and Albert Cane, Los Angeles, Calif., assignors to Hughes Tool Company, Aircraft Division, Houston, Tex., a corporation of Delaware N0 Drawing. Filed Sept. 1, 1960, Ser. No. 53,566 11 Claims. (Cl. 149-19) The present invention relates generally to gas-producing propellants suitable for use in thrust-producing rocxet engines, and more particularly to solid propellants of this character and to methods of making same.

In the art of rocketry, one of the most pressing needs for effecting propulsion of large scale rockets, is a highenergy solid propellant which is castable into a single massive grain of maximal specific impulse. Accordingly, it is the primary purpose of this invention to provide a rocket propellant of this general character, and to further provide such a propellant which can be prepared with comparative ease under conditions of optimum safety for producing homogeneous propellant grains of any desired size and shape having good qualities of stability with respect to shock, heat, atmospheric humidity, and ordinary handling.

To provide a propellant having the features outlined above, the invention contemplates a solid composition comprising finely divided particles of fuel and oxidizer materials evenly dispersed in a polymer of at least one monomer selected from the group of monomers consisting of fluoroalkyl acrylates and fluoroalkyl methacrylates characterized by the general formula in which n is an integer from 2 to and R is selected from the group consisting of H and CH Although fluoroalkyl acrylates and fluoroalkyl methacrylates can be used with various combinations of solid oxidizer and fuel materials for producing propellant compositions in different energy categories, the present invention is primarily concerned with production of highenergy propellants which employ solid oxidizer materials selected from the group consisting of perchlorates and nitrates respectively characterized by general formulas YCIO, and YNO in which Y is at least one member selected from the group consisting of NH N H N0 Li, Na and K, and which also employ solid fuel materials selected from the group consisting of aluminum, boron, beryllium, magnesium, lithium, aluminum, hydride, lith- 'ium hydride, lithium aluminum hydride, beryllium hydride, magnesium hydride, zirconium hydride, solid boron hydrides, and boron-containing compounds characterized by the general formula B H in which z/x is a numher from 2 to 8 and x/y is a number from 1 to 10, and by the general formulas B C H and B C H Z in which Z is an aliphatic radicalln the latter compound, it is preferred that the aliphatic radical Z contain not more than 4 carbon atoms.

There are numerous advantages in using fluoroalkyl acrylates and/or fluoroalkyl methacrylates as binder materials in compounding high-energy solid propellants. For example, these monomers are nonhydroscopic and are stable at room temperatures; they are physically and chemically compatible with the above-named fuel and oxidizer materials; they are capable of being polymerized either singly or in combination to a strong rubbery solid by a simple polymerization process; and because of their high fluorine content, they act as oxidizers in the compound such as to effect substantial reductions in the quantity requirements for separate and specific oxidizer materials. Moreover, the members of this group of monomers can be combined in various proportions to provide polymers having various physical properties such as different degrees of hardness, tensile strength etc. In addition, these monomers, used singly and/or in combination, are liquids; hence they provide a condition of fluidity which is essential to a castable compound. Regarding this fluidity, it is noted that, in both the acrylate and methacrylate series, the monomers in which n is 5, are solids at temperatures below 35 C., and that they are soluble in the liquid members of either series. Thus, when a solid member is dissolved in one or more of the liquid members the resulting solution is a liquid at room temperatures. However it should also be noted that such monomers in which n is 5 liquefy at temperatures above 35 (3.; hence, fuel and oxidizer materials can be readily mixed with these monomers providing the ingredients in the mixture are compatible with each other at the temperatures effecting liquidity.

In preparing the mixture or compound for casting a high-energy solid propellant grain according to this invention, at least one polymerizing catalyst from the family of organic peroxides preferably selected from the group consisting of benzoyl peroxide, tertiary butyl peroxide, and tertiary butyl perbenzoate is dissolved in at least one liquid monomer selected from the class consisting of fluoroalkyl acrylates and fluoroalkyl methacrylates; the amount of said catalyst being from about 0.005 percent to about 2 percent by weight of the monomer. When the solution is completed, the resulting catalystcontaining monomer is partially polymerized at a temperature of about 50 C. to increase the viscosity thereof before mixing said monomer with fuel and oxidizer materials, for reasons hereinafter described.

After cooling to room temperature, the partially polymerized monomer is thoroughly mixed With the dry fuel and oxidizer materials selected for the castable compound. During this operation, considerable heat may be evolved; hence, from a practical point of view, the mixing process is preferably performed under controlled conditions wherein the temperature of the mixture is maintained at about 20 C. It is essential however, that during the mixing operation, the mixture be maintained at a temperature safely below the ignition point of the fuel involved.

Regarding the partial polymerization, above, described, the increased viscosity of the monomer should be such as to assure a homogeneous mixture of the monomer material and the fuel and oxidizer materials, and at the same time, provide a fluid mass suitable for pouring a cast propellant grain wherein the fuel and oxidizer ingredients are evenly dispersed throughout the poured mixture. In other words, even though the finely divided solid particles of the fuel and oxidizer act as a thickener for the monomer, the viscosity of the latter should be such as to preclude settling of the solid particles after the mixture has been poured into a suitable mold.

Partial polymerization is also important in that during the mixing process a more viscous monomer acts to coat the separate particles of the fuel and oxidizer materials instead of being absorbed thereby, as would be the case with a less viscous monomer. Moreover, the coated particles are, in effect, insulated from undesired chemical activity during subsequent curing operations performed on the cast propellant grain. Attention is further directed to the fact that, of the fuel materials named above as being suitable for high-energy propellants, at least one; namely, lithium aluminum hydride is, of itself, a catalyst of the ionic type for fluoroalkyl acrylates and fluoroalkyl methacrylates. Thus, in a propellant composition according to this invention wherein the fuel material is lithium aluminum hydride, the catalystic action of this fuel material competes with the catalytic action of the catalyst from the organic peroxide family such as benzoyl peroxide, which latter catalytic action is of the preferred free radical type. This preference stems from the fact that the action of an ionic type catalyst is more rapid than the action of a free-radical type catalyst; hence, where polymerization of a monomer in a propellant mixture is effected by an ionic type catalyst, the time period of the mixture as a pourable liquid is of shorter duration than would be the case if the polymerization were effected by a catalyst of the free radical type. In other words, more time is available for mixing and pouring a propellant composition in which polymerization of the monomer is effected by a free-radical type catalyst.

From the foregoing comments concerning the catalytic properties of lithium aluminum hydride with respect to fluoroalkyl acrylate and methacrylate monomers, it is clearly apparent that an additional advantage results from the partial polymerization of these monomers, in that a catalytic mechanism of the free radical type is caused to be well established before fuel and monomer materials are intermixed; hence, the polymerizing activity of the lithium aluminum hydride is effectively inhibited.

In practicing this invention, propellant grains with satisfactory physical and performance characteristics have been cast from a number of different fluid propellant compounds consisting essentially of fuel and oxidizer materials intermixed in partially polymerized binder materials from the fluoroalkyl acrylate and methacrylate group of monomers. In these different compounds, the fuel materials selected from those previously named were varied in quantity from about 5 percent to about percent by weight of the whole mixture; while the oxidizer materials from the perchlorates and nitrates were varied in quantity from about percent to about 75 percent by weight of the whole mixture; whereas the catalyst-containing partially polymerized binder materials were varied in quantity from about 20 percent to about 50 percent by weight of the whole mixture. Each of the different compounds, after a thorough mixing under conditions precluding the temperature thereof from exceeding the upper limit of about 20 C., was poured into a rocket engine casing or other suitable mold where it was subjected to heat between the temperatures of about 35 C. to about 85 C. for a period of time between about 120 hours and 168 hours so as to cure the plastic binder material and thus provide a solid propellant grain.

The following examples are representative of the propellant compounds and resulting solid propellants referred to in the preceding paragraph.

Example I A one-pound batch was thoroughly mixed under controlled conditions wherein the temperature of the mixture was prevented from exceeding 20 C., which "batch consisted of the following ingredients in percentages of the total batch weight: 15 percent lithium aluminum hydride; 42 percent ammonium perchlorate; 0.10 percent carbon black; and 42.90 percent of a partially polymerized binder material consisting of, in percentages of the total binder material Weight, 99.85 percent of a fluoroalkyl methacrylate monomer in which n is 4 and R is CH and 0.15 percent of benzoyl peroxide. After the mixing operation, the resulting highly viscous mixture was poured into a rocket engine casing where it was heated in successive steps at a first temperature of 35 C. for a time period of 24 hours, a second temperature of 45 C. for a period of 24 hours, and a third temperature of 55 C. for a period of 120 hours. The resultant propellant grain, cast in situ, was a rubbery solid having good physical properties, which grain burned smoothly and completely in a successful firing.

Example II A ten-pound batch was thoroughly mixed under controlled conditions wherein the temperature was maintained well below the ignition point of the mixture. In this example the batch consisted of the following ingredients in percentages of the total batch weight: 20 percent pulverized aluminum; 55 percent ammonium perchlorate; and 25 percent of a partially polymerized binder material consisting of, in percentages of the total binder material weight, 59.91 percent of a fluoroalkyl methacrylate monomer in which n is 3 and R is CH 39.94 percent of a fluoroalkyl methacrylate monomer in which n is 4 and R is CH and 0.15 percent of benzoyl peroxide. After the mixing operation, the resulting highly viscous mixture was poured into a rocket engine casing where it was heated in successive steps at a first temperature of 50 C. for a time period of 72 hours, a second temperature of 70 C. for a period of 24 hours, and a third temperature of C. for a period of 24 hours. The resultant pro pellant grain, cast and cured in situ, was a rubbery solid having superior qualities in regard to stability, impact sensitivity, and mechanical properties. This propellant grain, successfully fired in a test stand, showed performance characteristics of the order indicated by theoretical considerations.

Example III A S-gram solid propellant grain was prepared according to Example I, except that the fuel ingredient lithium aluminum hydride was replaced with decaborane, one of the solid boron hydrides. The materials involved were wholly compatible throughout the mixing and curing operations. Moreover, the resultant product was a solid rubbery grain of good physical properties, which burned smoothly and completely when tested.

Example IV A 5-gram solid propellant grain was prepared according to Example I, except that oxidizer material nitronium perchlorate was used in place of ammonium perchlorate. In this example, the materials were fully compatible and produced a solid rubbery grain having good physical properties, which grain burned smoothly and completely when tested.

Example V A S-gram batch was thoroughly mixed at room temperature (about 20 C.). In this example, the batch consisted of the following ingredients, in percentages of the total batch weight: 20 percent pulverized aluminum; 55 percent ammonium perchlorate; and 25 percent of a partially polymerized binder material consisting of, in percentages of the total binder material weight, 99.90 percent of a fluoroalkyl acrylate monomer in which n is 2 and R is H, and 0.10 percent of benzoyl peroxide. After the mixing operation, the resulting highly viscous mixture was poured into a mold where it was heated at a temperature of 50 C. for a period of hours. The cured grain was a rubbery solid having a good mechanical properties and satisfactory burning qualities.

Example VI The batch in this example consisted of, in percentages of its total weight: 15 percent pulverized aluminum; 55 percent ammonium perchlorate; and 30 percent of a partially polymerized binder material consisting of, in percentages of the total binder material weight, 99.90 percent of a fluoroalkyl acrylate monomer in which n is 5 and R is H, and 0.10 percent of benzoyl peroxide. Inasmuch as the monomer in this example is a solid at temperatures below 35 C., the mixing operation was performed at a temperature of about 40 C., safely below the ignition point of the mixture. The batch When thoroughly mixed was poured into a suitable mold where it was subjected to more heat at a temperature of 50 C. for a period of 120 hours. The cured grain was a rubbery solid which, when tested, showed good physical properties and acceptable burning qualities.

In using fluoroalkyl acrylates and fiuoroalkyl methacrylates as binder materials for solid propellants, another advantage results from the fact that the fluorine content of the homologues in both series varies from 50.6 percent to 65.0 percent; hence a binder material having any desired fluorine content between these percentages may be provided by mixing different homologues from either or both series. In this way, cast propellant grains of different physical properties can be produced. Moreover, these monomers are of low volatility and present no toxicity or odor problems; they are not sensitive to moisture, except in mixtures where another ingredient such as lithium aluminum hydride or nitronium perchlorate precludes the presence of moisture; and when polymerized as binders they are inert to a wide variety of reactive chemicals, and have demonstrated high degrees of stability to heat under short exposure and prolonged aging conditions.

What is claimed is:

1. A solid rocket propellant grain comprising: from about percent to about 25 percent by weight, of pulverized solid fuel material; from about 35 percent to about 75 percent by weight, of pulverized solid oxidizer material; and from about 20 percent to about 50 percent by weight, of solidified binder material in which said fuel and oxidizer materials are uniformly dispersed; said binder material consisting essentially of a polymer of at least one monomer selected from the group consisting of fluoroalkyl acrylates and fluoroalkyl methacrylates characterized by the general formula in which n is an integer from 2 to 5 and R is selected from the group consisting of H and CH said fuel material consisting of at least one member selected from the group consisting of aluminum, boron, beryllium, magnesium, lithium, aluminum hydride, lithium hydride, lithium aluminum hydride, beryllium hydride, magnesium hydride, zirconium hydride, solid boron hydrides, and boron-containing compounds characterized by the general formula B N H in which z/x is a number from 2 to 8 and x/y is a number from 1 to 10, and by the general formulas B C H and B C H Z in which Z is an alphatic radical; and said oxidizer material consisting of at least one member selected from the group consisting of perchlorates and nitrates respectively characterized by general formulas YClO and YNO in which Y is at least one member selected from the group consisting of NH N2H3, N02, Na and K.

2. A solid rocket propellant grain according to claim 1 in which the pulverized solid fuel material consists of at least one member selected from the group consisting of aluminum, boron, beryllium, magnesium, lithium, aluminum hydride, lithium aluminum hydride, beryllium hydride, magnesium hydride, zirconium hydride, and solid boron hydrides.

3. A solid rocket propellant grain according to claim 2 in which the pulverized solid oxidizer material consists of at least one member selected from the group consisting of ammonium perchlorate and nitronium perchlorate.

4. A solid rocket propellant grain according to claim 3 wherein the pulverized solid fuel material consists of at least one member selected from the group consisting of aluminum, lithium aluminum hydride and decaborane; wherein the pulverized solid oxidizer consists essentially of ammonium perchlorate; and wherein the solidified binder material consists essentially of a polymer of at least one monomer selected from the group consisting of 6 fluoroalkyl acrylates characterized by the general formula i 11(0 FzCH2)nCH2-O -C=CH2 in which n is an integer from 2 to 5.

5. A solid rocket propellant grain according to claim 4 in which the pulverized solid fuel material consists essentially of aluminum; and in which the solidified binder material is a polymer of the fluoroalkyl acrylate monomer characterized by the formula 6. A solid rocket propellant grain according to claim 4 in which the pulverized solid fuel material consists essentially of aluminum; and in which the solidified binder material is a polymer of the fluoroalkyl acrylate monomer characterized by the formula p 7. A solid rocket propellant grain according to claim 3 wherein the pulverized solid fuel material consists of at least one member selected from the group consisting of aluminum, lithium aluminum hydride, and decaborane; and wherein the solidified binder material consists essentially of a polymer of at least one monomer selected from the group consisting of fiuoroalkyl methacrylates characterized by the general formula H(CFzCF2)nCHzO i-0:013,

CH3 in which n is an integer from 2 to 5.

8. A solid rocket propellant grain according to claim 7 wherein the pulverized solid fuel material consists essentially of aluminum; wherein the pulverized solid oxidizer material consists essentially of ammonium perchlorate; and wherein the solidified binder material consists of a copolymer of monomers selected from the group consisting of fluoroalkyl methacrylates characterized by the general formula H3 in which n is an integer from 3 to 4.

9. A solid rocket propellant grain according to claim 7 wherein the pulverized solid fuel material consists essentially of lithium aluminum hyride; wherein the pulverized solid oxidizer material consists essentially of ammonium perchlorate; and wherein the solidified binder material consists essentially of a polymer of a fluoroalkyl methacrylate monomer characterized by the formula 10. A solid rocket propellant grain according to claim 7 wherein the pulverized solid fuel material consists essentially of decaborane; wherein the pulverized solid oxidizer material consists essentially of ammonium perchlorate; and wherein the solidified binder material consists essentially of a polymer of a fiuoroalkyl methacrylate monomer characterized by the formula 11. A solid rocket propellant grain according to claim 7 wherein the pulverized solid fuel material consists essentially of lithium aluminum hydride; wherein the pul- 7 8 verized solid oxidizer material consists essentially of OTHER REFERENCES nitronium perchlorate; and wherein the solidified binder Father Astronautics August 1960 pp 34 and 42 material consists essentially of a polymer of a fluoroalkyl Smith Memorandunl NO 20478 EiastgmerioBindr methacrylate monomer characterized by the formula and MelzhanicabProperty liequiremgnts for Solid pellants, Jet Propulsion Laboratory, Calif., Inst. Tech., H(CF2-CF2) OH2O( JC=CH2 Pasadena, Calif., Jan. 7, 1959, pp. 1, 2, 11, 12 and 13. Warren; Rocket Propellants, Reinhold Publishing Corp., New York (1958), pp. 60-67. References Cited UNITED STATES PATENTS 10 BENJAMIN R. PADGETT, Primary Examiner. 2,622,277 12/1952 Bonell et a1. LEON D. ROSDOL, ROGER L. CAMPBELL, 3,203,171 8/1965 Burke et a1 14919 Examiners. 

1. A SOLID ROCKET PROPELLANT GRAIN COMPRISING: FROM ABOUT 5 PERCENT TO ABOUT 25 PERCENT BY WEIGHT, OF PULVERIZED SOLID FUEL MATERIAL; FROM ABOUT 35 PERCENT TO ABOUT 75 PERCENT BY WEIGHT, OF PURLVERIZED SOLID OXIDIZER MATERIAL; AND FROM ABOUT 20 PERCENT TO ABOUT 50 PERCENT BY WEIGHT, OF SOLIDIFIED BINDER MATERIAL IN WHCIH SAID FUEL AND OXIDIZER MATERIALS ARE UNIFORMLY DISPERSED; SAID BINDER MATERIAL CONSISTING ESSENTIALLY OF A POLYMER OF AT LEAST ONE MONOMER SELECTED FROM THE GROUP CONSISTING OF FLUOROALKYL ACRYLATES AND FLUOROALKYL METHACRYLATES CHARACTERIZED BY THE GENERAL FORMULA 